Change NotImplementedException to UnimplementedError.

lib/compiler/implementation/lib/isolate_patch.dart

Index: lib/compiler/implementation/lib/isolate_patch.dart
diff --git a/lib/compiler/implementation/lib/isolate_patch.dart b/lib/compiler/implementation/lib/isolate_patch.dart
index 58b8093692950567c4e21fe89d54409d7a74f43c..b71e9cf9bf3d570798e74bbc8406a058b19328eb 100644
--- a/lib/compiler/implementation/lib/isolate_patch.dart
+++ b/lib/compiler/implementation/lib/isolate_patch.dart
@@ -378,7 +378,7 @@ abstract class _ManagerStub {
 /** A stub for interacting with the main manager. */
 class _MainManagerStub implements _ManagerStub {
   get id => 0;
-  void set id(int i) { throw new NotImplementedException(); }
+  void set id(int i) { throw new UnimplementedError(); }
   void set onmessage(f) {
     throw new Exception("onmessage should not be set on MainManagerStub");
   }